Regulations established by the U.S. Federal Aviation Administration (FAA), Transport Canada and other international counterparts prohibit the takeoff of any aircraft when frost, ice or snow is adhered to aircraft wings, propellers or control surfaces. Regulations also prohibit takeoff at any time that frost, ice or snow can reasonably be expected to adhere to the aircraft, unless the operator has an approved de-icing or anti-icing program.
Common practice developed in North American and European aviation communities is to de-ice the accumulated snow and other freezing precipitates followed by anti-icing the aircraft before takeoff. The purpose of aircraft de-icing and anti-icing is to keep airplane wings and other aerodynamically sensitive areas clean before and during takeoff.
Aircraft de-icing is performed to clean aircraft surfaces of already accumulated freezing precipitates, by applying heated aircraft de-icing fluids. Commercial de-icing fluids are commonly glycol solutions such as ethylene glycol or propylene glycol water mixtures, which act as freezing point depressants.
Aircraft anti-icing is a preventive procedure which provides protection against the ongoing formation of frost or ice and accumulation of snow or slush on treated surfaces of the aircraft. Aircraft anti-icing is done by applying aircraft anti-icing fluids (AAF) after the aircraft is de-iced. Anti-icing fluids are often glycol solutions thickened with water soluble polymers. The typical concentrations for glycol solutions and water soluble polymers in AAF are 40-95% and 0.1-0.5% by weight, respectively. AAF is usually applied unheated to form a temporary protective layer on aircraft surfaces. The AAF layer has a lower freezing point than the frozen precipitation, which melts upon contact with the anti-icing fluid.
The water soluble polymer thickener keeps the anti-icing fluid from draining off sloped aircraft surfaces. This is important in keeping the aircraft free from precipitation while the aircraft waits for takeoff, without having to repeatedly apply the anti-icing fluid. However, quite often as the fluid layer becomes diluted by melting precipitation, its viscosity drops and the anti-icing fluid layer becomes thinner and less effective and frozen precipitate begins to accumulate.
Conversely, an anti-icing fluid that is too thick will cling to the aircraft surfaces even at high shear rates, such as those seen during takeoff. This can lead to unstable aerodynamics and have a detrimental effect of aircraft lift and balance.
It is therefore extremely important to develop a thickener for aircraft anti-icing fluids that retains thickness and viscosity during low shear conditions, such during taxiing and waiting, but which also shears off easily in high shear conditions, namely at takeoff. Such non-Newtonian fluids should advantageously change viscosity in varying shear conditions.